1. Field of the Invention
The present invention relates to semiconductor devices, and more particularly to a semiconductor device that includes an output impedance controller for controlling impedance of an output buffer according to an impedance value of an external resistor.
2. Discussion of Related Art
Many electronic products, for example, personal computers and workstations, include various semiconductor devices such as microcomputers, memories, and gate arrays. The semiconductor devices typically include input/output pins and an output circuit for data transmission with other devices. The output circuit has, for example, output buffers and drivers. The input/output pins of the semiconductor device are connected to corresponding transmission lines such as wires formed on a board. Internal data of a semiconductor device is provided to another semiconductor device through transmission lines that form an interface. For data output from a semiconductor device, the data is transmitted over the transmission lines, and for there to be proper transmission, impedance between the input/output pins and the transmission lines needs to be matched.
As operational speeds of electronic products increase, the signal swing transmitted through an interface decreases. This allows a delay time to be minimized at signal transmission. However, as the signal swing width of a transmitted signal decreases, external noise increases. External noise can affect the reflection characteristics output signal if there is impedance mismatch at an interface. Impedance mismatch is caused by external noise or by power supply voltage, temperature and process variations. If impedance mismatch arises, a transmission speed of the data decreases, and the data from a semiconductor device is distorted. Thus, in a case where a semiconductor device receives distorted data, problems can be caused by setup/hold failures or errors in reading received data.
Impedance correcting techniques for matching impedance are used for semiconductor memory devices with other devices. For example, in case of an HSTL (high speed transceiver logic) interface, a technique exists for controlling impedance so as to have a desired output impedance value within several tens of ohms using one additional pin. In case of a semiconductor device adopting the impedance correcting techniques of semiconductor memory devices, however, it is difficult to obtain output impedance equal to a designed value because of variations in power supply voltage, temperature, and processing.
To overcome aforementioned problems, generally, a trimming circuit having a metal, bonding, or fuse operation is provided in a semiconductor device. Impedance matching is accomplished by adjusting a reference voltage for impedance matching and a resistance value range using the trimming circuit. This technique is called a programmable impedance control function. However, the programmable impedance control function technique needs a considerable amount of test time and an additional process for adjusting impedance. As a result, fabrication costs can increase.
Accordingly, a need exists for a system and method that can obtain a desired output impedance value irrespective of power supply voltage, temperature, and process variations to match impedance.